Evaluation of the coefficient of lateral stress at rest of granular materials under repetitive loading conditions

Although the internal stress state of soils can be affected by repetitive loading,there are few studies evaluating the lateral stress(or K_(0))of soils under repetitive loading.This study investigates the changes in K_(0) and directional shear wave velocity(V_(s))in samples of two granular materials with different particle shapes during repetitive loading.A modified oedometer cell equipped with bender elements and a diaphragm transducer was developed to measure the variations in the lateral stress and the shear wave velocity,under repetitive loading on the loading and unloading paths.The study produced the following results:(1)Repetitive loading on the loading path resulted in an increase in the K_(0) of test samples as a function of cyclic loading number(i),and(2)Repetitive loading on the unloading path resulted in a decrease in K_(0) according to i.The shear wave velocity ratio(i.e.V_(s)(HH)/V_(s)(VH),where the first and second letters in parentheses corresponds to the directions of wave propagation and particle motion,respectively,and V and H corresponds to the vertical and horizontal directions,respectively)according to i supports the experimental observations of this study.However,when the tested material was in lightly over-consolidated state,there was an increase in K_(0) during repetitive loading,indicating that it was the initial K_(0),rather than the loading path,which is responsible for the change in K_(0).The power model can capture the variation in the K_(0) of samples according to i.Notably,the K_(0)=1 line acts as the boundary between the increase and decrease in K_(0) under repetitive loading.

Cyclic Lateral Responses of Monopiles Considering the Influence of Pile−Soil Relative Stiffness in Sand

The existing studies have primarily focused on the effect of cyclic load characteristics(namely,cyclic load ratio and amplitude ratio)on cyclic lateral response of monopiles in sand,with little attention paid to the effect of pile−soil relative stiffness(K_(R)).This paper presents a series of 1-g cyclic tests aimed at improving understanding of the cyclic lateral responses of monopiles under different pile−soil systems.These systems are arranged by two model piles with different stiffness,including four different slenderness ratios(pile embedded length,L,normalized by diameter,D)under medium dense sand.The K_(R)-values are calculated by a previously proposed method considering the real soil stress level.The test results show that the lateral accumulation displacement increases significantly with the increment of the K_(R)-value,while the cyclic secant stiffness performs inversely.The maximum pile bending moment increases with the cycle number for the rigid pile−soil system,but shows a decreasing trend in the flexible system.For an uppermost concern,an empirical model is proposed to predict the accumulated displacement of arbitrary pile−soil systems by combining the results from this study with those from previous experimental investigations.The validity of the proposed model is demonstrated by 1-g and centrifuge tests.

Lateral magnetic stiffness under different parameters in a high-temperature superconductor levitation system

Magnetic stiffness determines the stability of a high-temperature superconductor(HTS)magnetic levitation system.The quantitative properties of the physical and geometrical parameters that affect the stiffness of HTS levitation systems should be identified for improving the stiffness by some effective methods.The magnetic stiffness is directly related to the first-order derivative of the magnetic force with respect to the corresponding displacement,which indicates that the effects of the parameters on the stiffness should be different from the relationships between the forces and the same parameters.In this paper,we study the influences of some physical and geometrical parameters,including the strength of the external magnetic field(B0)produced by a rectangular permanent magnet(PM),critical current density(Jc),the PM-to-HTS area ratio(α),and thickness ratio(β),on the lateral stiffness by using a numerical approach under zero-field cooling(ZFC)and field cooling(FC)conditions.In the first and second passes of the PM,the lateral stiffness at most of lateral positions essentially increases with B0 increasing and decreases withβincreasing in ZFC and FC.The largest lateral stiffness at every lateral position is almost produced by the minimum value of Jc,which is obviously different from the lateral force–Jc relation.Theα-dependent lateral stiffness changes with some parameters,which include the cooling conditions of the bulk HTS,lateral displacement,and movement history of the PM.These findings can provide some suggestions for improving the lateral stiffness of the HTS levitation system.

Elastohydrodynamic Lubrication Interface Stiffness and Damping Considering Asperity Lateral Contact

Elastohydrodynamic lubrication(EHL)point contact occurs between two rough surfaces at the mesoscopic level,while the interaction of rough surfaces involves contact between asperities at the microscale level.In most cases,the contact between asperities within an interface takes the form of lateral contact rather than peak contact.Regions devoid of contact asperities are filled with lubricating oil.However,conventional models often oversimplify lateral contact forms as interactions between asperities and a smooth,rigid plane.These simplifications fail to accurately represent the true contact conditions and can lead to inaccuracies in the analysis of EHL’s contact performance.To address this issue,we have developed a novel EHL interface model comprising two rough surfaces.This model allows us to explore the influence of asperity height,contact angle,and contact azimuth angle on EHL interface performance.

Simplified stiffness-based approach for seismic performance evaluation of moment-resisting frame

Based on the concept of stiffness degradation, a damage index of the whole frame and the storey is proposed for the frame seismic performance evaluation. The index is compatible with the non-linear static analysis （e. g. the pushover analysis）, and the structural damage is considered via plastic hinges. Simultaneously, a practical approach is developed to obtain the relationships between the proposed index and earthquake intensities based on the capacity spectrum method. The proposed index is then illustrated through two low-rise reinforced concrete frames, and it is also compared with some other indices. The results indicate that the proposed index is on the safe side and not sensitive to the lateral load pattern. The storey index is helpful to reflect the storey damage and to uncover the position of the weak storey. Finally, the relationship between performance levels and damage index values is also proposed through statistical analysis for the performance-based seismic evaluation.

Behavior of Pile Group with Elevated Cap Subjected to Cyclic Lateral Loads

The pile group with elevated cap is widely used as foundation of offshore structures such as turbines, power transmission towers and bridge piers, and understanding its behavior under cyclic lateral loads induced by waves, tide water and winds, is of great importance to designing. A large-scale model test on 3×3 pile group with elevated cap subjected to cyclic lateral loads was performed in saturated silts. The preparation and implementation of the test is presented. Steel pipes with the outer diameter of 114 mm, thickness of 4.5 mm, and length of 6 m were employed as model piles. The pile group was cyclic loaded in a multi-stage sequence with the lateral displacement controlled. In addition, a single pile test was also conducted at the same site for comparison. The displacement of the pile cap, the internal forces of individual piles, and the horizontal stiffness of the pile group are presented and discussed in detail. The results indicate that the lateral cyclic loads have a greater impact on pile group than that on a single pile, and give rise to the significant plastic strain in the soil around piles. The lateral loads carried by each row of piles within the group would be redistributed with loading cycles. The lateral stiffness of the pile group decreases gradually with cycles and broadly presents three different degradation patterns in the test. Significant axial forces were measured out in some piles within the group, owing to the strong restraint provided by the cap, and finally lead to a large settlement of the pile group. These findings can be referred for foundation designing of offshore structures.

An Inelastic Material Model for Lateral Torsional Buckling and Biaxial Bending of Steel W-Shapes

A new material model for beam elements was developed for use as normalized tangent modulus expressions when performing 3-dimensional second-order inelastic analyses of steel I-section beams.The stiffness matrix of a 14 degree-of-freedom beam element was updated to include the effects of yielding on St.Venant’s torsion and bimoment stiffness at the initial and terminal nodes.A validation study compared the new model’s results with those from published detailed finite element analyses and was found to be in very close agreement.A biaxial end-moment study with two different depth-to-flange-width ratios provided expected and consistent results over a range of moment conditions.

道砟非均匀分布对道床力学特性的影响

铁路道床机械清筛作业中,分次回填道砟颗粒会导致道砟分层,出现非均匀分布现象。为了解道砟非均匀分布对道床力学特性的影响,基于离散单元法基本原理,生成有砟道床仿真模型,模拟道砟均匀分布、“上大下小”分层分布、“上小下大”分层分布等3种道床工况,分析3种道床工况在捣固作业和稳定作业中的力学特性变化。结果表明:道砟非均匀分布对道床力学特性产生影响,其中道床密实度特性受道砟分布状态影响较小,而道床垂向刚度和横向阻力特性则受道砟分布状态影响较大。在捣固作业中,道砟“上小下大”分层分布最有利于道床垂向刚度提升。在稳定作业中,道砟“上大下小”分层分布最有利于道床横向阻力提升。道砟颗粒流动则是使得道床垂向刚度和横向阻力提升的重要因素。

基于六点刚度试验的输送带翘曲问题分析

圆管带式输送机具有封闭运输的优点,但相较于普通带式输送机也存在一些缺点,如输送带横向刚度引起的输送带翘曲问题。文中针对输送带翘曲问题,基于六点刚度试验进行输送带仿真模型的建立,并通过仿真模型对输送带翘曲现象进行了定性分析。结果表明,输送带的翘曲量与输送带的横向刚度呈非线性增加的关系,与张力呈非线性减小的关系,并且随托辊间距的增加而增加,本研究为输送带的选型和圆管带式输送机的设计提供一定的参考。

分布式双电机后驱电动汽车的横摆稳定性控制策略

【目的】为了提高车辆的横摆稳定性,提出了一种三层结构的分布式双电机后驱电动汽车横摆稳定性控制策略。【方法】上层观测器采用扩展Kalman滤波器估计动态前后轴的侧偏刚度,以提高控制系统的模型精度与稳定性;中层控制器采用基于正切函数的滑模控制器对系统误差进行控制,通过调节系统误差调节期望的附加横摆力矩的大小;下层控制器以轮胎利用率之和最小为优化目标,将期望的附加横摆力矩分配到后轴驱动轮上,从而实现弯道行驶过程中车辆的横摆稳定性控制。【结果】仿真结果表明,相比于采用静态侧偏刚度的横摆稳定性控制系统,最大滑模面误差减少了44%,横摆角速度期望值趋近于实际值的程度提高了10.5%。【结论】本文设计的控制策略具有更好的鲁棒性,同时动态侧偏刚度使横摆角速度期望模型的期望值更趋近于实际横摆角速度值。

四边连接圆形压痕钢板剪力墙的抗侧能力分析

为合理调节或优化传统钢板剪力墙(steel plate shear wall,SPSW)的抗震性能,本文提出了一种新型的带圆形压痕的钢板剪力墙结构(steel plate shear wall with circle dents,SPSW-CD)。基于有限元方法系统研究了压痕孔径、压痕孔距、压痕深度、钢板厚度和钢材强度等参数对SPSW-CD抗侧性能的影响,提出了四边连接SPSW-CD抗侧刚度及水平承载力的简化计算公式。分析结果表明随着压痕孔径增加,SPSW-CD的初始抗侧刚度及水平承载力均呈降低趋势;压痕孔距及钢板厚度越大,SPSW-CD抗侧性能越优。随着钢材强度增加,SPSW-CD水平承载力呈明显的增大趋势,但对其抗侧刚度影响不明显。压痕深度对SPSW-CD抗侧性能的影响可忽略。SPSW-CD抗侧刚度及水平承载力计算公式计算误差均小于10%,研究成果可用于此类结构的工程设计。

基于凸轮-滚子结构的多自由度准零刚度系统设计

基于凸轮-滚子结构设计一种多自由度非线性准零刚度低频隔振装置。通过静力学特性计算,结合横向刚度计算理论,得到系统在多个自由度上实现准零刚度的参数条件。利用谐波平衡法分析了系统动力学特性,得出系统力传递率,并分析了预压、阻尼比及激励力对系统振动传递率的影响。为验证准零刚度系统隔振性能,搭建隔振测试系统,试验结果表明,相较于传统线性隔振系统,多自由度隔振系统在竖直方向上起始隔振频率从6.7 Hz降低至3.5 Hz,隔振效率最大提升约84.3%;水平方向上起始隔振频率从6.0 Hz降低至4.0 Hz,隔振效率最大提升约45.0%;在10.0 Hz以下,多自由度隔振系统的振动传递率远小于线性隔振系统。

重载铁路用新型网孔式弹性垫板的力学特性研究

重载铁路轨道系统的弹性垫板由于承受载荷较大,易出现疲劳破损现象。鉴于既有弹性垫板存在应力集中且阻尼较小的现象,提出一种新型网孔式弹性垫板,可改善应力分布,提升阻尼特性。为探究将网孔式弹性垫板应用于重载铁路扣件系统的可行性并验证相较于既有弹性垫板的优越性,分别建立适用于重载铁路弹条Ⅱ型扣件系统的既有弹性垫板和新型网孔式弹性垫板有限元模型。在同刚度条件下,对比研究网孔式、沟槽式、棱台式弹性垫板的竖向刚度、横向刚度和应力分布特征。研究结果表明,网孔式弹性垫板的应力应变分布更均匀,其最大应力相较于沟槽式弹性垫板和棱台式弹性垫板降低约30%,力学特性更好,且具有较好的竖向刚度保持性能;在施加横向载荷后,网孔式弹性垫板的横向刚度比棱台式弹性垫板的小,更接近于沟槽式弹性垫板;新型网孔式弹性垫板具有在重载铁路中推广应用的潜力。

离散侧向平移支撑圆弧拱的自由振动分析

拱通常采用离散侧向平移支撑增加其面外稳定性,而离散侧向平移支撑会产生振型耦合效应,显著影响拱的振动特性。为研究等间距离散侧向平移支撑对两铰圆弧拱自由振动的影响,通过假定拱的耦合振型,基于Hamilton原理求解离散侧向平移支撑圆弧拱的自振频率,并与有限元数值结果比较。同时,还探究了拱的面外长细比、圆心角以及侧向支撑的数量和位置对拱的临界刚度及最大基频的影响。研究结果表明:假定的耦合振型可以描述复杂的离散侧向平移支撑拱的弯扭振型,并能准确预测拱的自振频率和支撑临界刚度。侧向平移支撑拱的基频随支撑刚度的增加呈现多种增长模式,根据振型变化情况将其划分为第1类、过渡和第2类增长模式,不同增长模式之间的转换取决于拱的面外长细比、拱的圆心角、支撑数量以及支撑位置。当拱的面外长细比和拱的圆心角的组合靠近第1类增长模式和其他增长模式的分界线时,拱的支撑临界刚度和最大基频显著增大。支撑位置靠近两端翼缘时可能会改变拱的基频振型,但支撑位置在剪切中心附近时,拱的基频最大。拱基频在第1类增长模式时,支撑数量的增加显著提高拱的最大基频,在过渡增长模式时,支撑数量的增加对最大基频的影响减弱,而在第2类增长模式下,拱的最大基频随支撑数量的增加基本不发生变化。

考虑系梁作用高墩刚构桥整体稳定的实用算法

研究目的:高墩刚构桥柱式高墩间常常设置系梁,目前计算高墩刚构桥整体稳定主要有计算长度系数法和有限元法,计算长度系数法须逐根桥墩计算且无法考虑不同系梁间桥墩段的影响,有限元法由于计算理论和方法都隐含在软件中,如何对计算结果的准确性和可靠性进行验证又是必须解决的难题。为此,本文基于摇摆柱受力特点通过结构转换的概念将确定柱式高墩刚构桥整体稳定的二阶计算转化为确定其整体抗侧刚度的一阶问题。研究结论:(1)本文提供了一种通过结构抗侧刚度求解高墩刚构桥整体稳定临界力的计算方法;(2)本文给出了柱式高墩刚构桥整体稳定临界力的计算公式,该式能考虑柱墩之间系梁的影响,弥补了规范计算长度系数法的不足,具有很好的精度及准确性,可为校核有限元整体稳定计算结果的可靠性提供一种解析验证手段;(3)本文研究成果可供桥梁工程设计和理论计算使用。

外侧柱入路肘关节松解术治疗肘关节僵硬的应用疗效研究

目的探究外侧柱入路手术松解治疗肘关节僵硬的应用及早期临床疗效。方法回顾性分析中国科学技术大学附属第一医院(安徽省立医院)关节外科2022年2月至2023年6月采用外侧柱入路肘关节松解术治疗肘关节僵硬患者的临床资料21例。其中,男15例,女6例;年龄12~61岁,平均(49.4±13.0)岁。采用Mayo肘关节功能(Mayo elbow performance score,MEPS)评分、肘关节活动度(range of motion,ROM)的变化及疼痛视觉模拟评分(visual analogue scale,VAS)对临床治疗效果进行评估。结果术后21例患者均获得随访,随访时间为3~16个月,平均(9.3±3.1)个月,患者的手术时间为39~155 min,平均(84.2±25.5)min,住院时间为3~16 d,平均(8.8±3.0)d,术中无血管、神经损伤。末次随访时所有患者肘关节MEPS评分、肘关节ROM的变化及VAS评分与术前相比均获得显著变化,差异存在统计学意义(P<0.05)。结论采用外侧柱入路肘关节松解术治疗肘关节僵硬可显著改善早期肘关节功能及活动度,为肘关节僵硬患者的手术治疗提供了一种安全且有效的术式选择。

多层剪切型支撑钢框架整体稳定性的解析算法研究

剪切型支撑钢框架整体稳定计算需考虑钢框架与剪切型支撑之间的相互作用,还需要考虑同层柱间的相互支援以及层与层之间的支援作用,规范计算长度系数法无法考虑这些因素可能会造成不合理设计。提出了一种计算多层剪切型支撑钢框架整体稳定性的解析算法,首先分析了支撑刚度与结构临界承载力之间的关系,推导了层临界支撑刚度计算公式,接着利用弹簧-摇摆柱力学模型推导了任意支撑下框架柱临界刚度比系数计算公式,然后将各楼层的有效抗侧刚度及荷载刚度进行楼层间的组装,将求解剪切型支撑钢框架的临界承载力转化为求解结构的楼层有效抗侧刚度,最后基于轴力权重加权平均的方法推导了可直接求解多层剪切型支撑钢框架临界承载力的计算公式,该公式能够判断结构的薄弱层,可以定量地计算楼层之间相互支援程度,有效地弥补规范尚无法求解弱支撑钢框架柱计算长度系数的不足。

基于Halbach阵列的永磁悬浮车辆悬浮系统磁力特性研究

提出对Halbach阵列永磁体进行尺寸参数优化,研究不同参数对永磁悬浮系统横向刚度问题的影响。以斥力型跨座式永磁悬浮车辆的悬浮永磁体模块为例,运用正弦法对悬浮永磁体的Halbach阵列磁场进行解析,利用Maxwell方法推导出永磁体的横向力和悬浮力随横向位移变化的公式,确定要优化的三个尺寸参数分别为:永磁体厚度、永磁体宽度和相邻两块永磁体的磁化角度差。利用软件COMSOL建立磁力模型,分析永磁悬浮模型在发生横向位移时,不同尺寸参数对模型产生的横向力和横向刚度的影响。仿真分析表明:增大永磁体的厚度会增大永磁悬浮系统的最大横向力和横向刚度;永磁体宽度的改变在一定范围内对永磁悬浮系统的横向刚度影响较小;减小相邻两块永磁体的磁化角度差,会在减小最大横向力的同时减小横向刚度,角度从90°减小到45°,横向刚度减小了58.4%。所得结论对永磁悬浮式磁浮车辆悬浮永磁体的设计和选择提供了一定参考。

子午线轮胎静态力学特性的有限元仿真

以205/55R18子午线轮胎为例,采用有限元仿真方法分析子午线轮胎的静态力学特性。建立有限元模型,并验证该模型的可靠性,在此基础上分析径向负荷、充气压力和接地摩擦因数对轮胎接地压力、纵向刚度和横向刚度的影响。结果表明:轮胎的径向负荷增大时,轮胎的接地压力、纵向刚度和横向刚度相应增大;轮胎的充气压力增大时,轮胎的接地面积减小,接地压力和刚度增大;通过增大摩擦因数可以提高轮胎的接地性能和操控性能,但摩擦因数的影响程度相比于径向负荷和充气压力较小。

车辆轨迹模型预测控制器的算法研究

为提高无人驾驶汽车行驶控制中的跟踪精度和车辆稳定性,提出了一个改进的模型预测控制器(MPC)。首先,为解决高速转弯工况下轮胎侧偏刚度的强非线性特性导致的车辆失稳问题,建立了轮胎动态侧偏刚度估计器进行实时估计;在此基础上,利用改进粒子群算法对现有模型预测控制器的时域参数进行优化,以提高算法跟踪精度和稳定性;最后,为验证控制器的效果,选取合适的工况进行了联合仿真测试。结果表明,改进控制器在高速转弯的工况下,跟踪精度误差降为4.9%,横摆角误差降为2.6%,比传统模型预测控制器分别提高了67.8%和62.3%,比模糊控制算法优化的控制器分别提高了55.8%和58%。